Systems are known in the art where local functionality can be controlled remotely. Such prior art systems are occasionally referred to as “slave” or “drop down” systems, in which the remote activation allows a remote operator to activate certain local functionality without having to be physically present at the local site.
In order to enable such remote control over local functionality, it is also conventional for each local site to have “mode” selection available, where an operator may ordain whether the local functionality may be controlled locally, or remotely, or shut down. In most conventional systems, remote mode will need to be selected to enable remote activation of the local functionality.
The example of tanks at a water treatment plant is illustrative. Typically there will be valve actuators located at the top of tanks and the plant operation level is at the bottom of the tanks. Alternatively, the valve actuators may be located at the bottom of the tanks and the plant operations level is at the top of the tanks, or at various platform elevations in the surrounding plant.
Each actuator will normally have a local control site physically attached to the actuator itself, where an operator may typically select one of three modes for valve actuation. These modes are conventionally referred to as “local,” “remote” and “off.” In local mode, the operator may cause the actuator to, for example, (1) open the valve, (2) close the valve, or (3) stop during an open or close activity, all from the local site. In local mode, actuator controls on the actuator itself become “hot”, allowing the operator to physically operate the actuator at the local site.
In remote mode, the local site will typically allow the operator to ordain that valve actuator operations such as “open”, “close” or “stop” at that site may be controlled from a remote location via, for example, a distributed control system (DCS). In remote mode, the local controls on the actuator itself are disabled, and control of the actuator is given to some remote control center connected to the actuator.
In off mode, the local site will typically allow the operator to shut down valve actuator operations at that local site (i.e., the actuator will not work again until either local or remote mode is re-selected).
Remote replication of such local mode selection is also advantageous. In the above example, if mode selection is also provided on the ground near the base of the tank, operators may select or change the mode of the valve actuator without climbing the tank. Further, co-location of remote access to several actuators on the ground, or perhaps at a control site, will allow the operator to select modes of those actuators in a coordinated fashion.
In addition to convenience, there are good reasons to provide remote replication of local actuator mode selection in plants generally. There may be a of loss of communication with the DCS, failure of the DCS, or other emergencies requiring operators to override DCS control. There is also always a requirement for maintenance. In these situations, remote replication of local mode control becomes advantageous.
It is common practice for plant design to include expensive platforms, catwalks, etc. with stairs or ladders just to allow plant operators physical access valve actuators at the actuator site in case of such emergencies or maintenance. It is typical in plants to find valves located above or below floor level requiring special equipment (such as ladders, scaffolding, skyclimbers, etc.) just to access local controls of the valve actuator. Of course, physical access to the actuators cannot be totally eliminated. In the case of a power failure, for example, access to hand wheel operation is required at some critical valves. This does not diminish the advantages provided by remote replication of local valve actuator functions such as mode control.
Hardwiring is a common conventional technique for enabling remote replication of local functionality. It is conventional for local function activators to be hardwired to their remote counterparts so that the function can be activated at either location. In the example of local/remote valve actuator control described above, it will be seen that two separate subsystems require hardwiring if this conventional technique is to be used. First, remote control of local mode selection needs hardwiring. Secondly, remote control of the valve actuator operations (e.g., open, close, stop) also needs hardwiring to allow such remote control in the event that remote mode is also selected. Such complicated hardwiring is expensive to design, install and maintain.
In fact, conventional valve actuator control systems provide only one hardwired selector switch. The single selector switch may be located physically at the local valve actuator, or at a remote replication site, but not both. While it is technically possible to construct a system with two hardwired selector switches, the number of interconnected wires increases to the point where the cost of the actuator is unreasonably inflated and/or functionality is lost. Having only one selector switch requires its physical site to be chosen during plant design and oftentimes prior to procurement of the valve actuator. Uncertainty as to the location of optimal actuator control is thus introduced.
There is a need in the art to simplify the implementation of at least remote replication of local mode control. In the example of valve actuation as described above, optimizations in plant control costs could be obtained if, without hardwiring, mode selection control could be replicated in at least one or two additional locations remote from the local actuator site, such as at the base of the tank and/or at some interim co-located control site. Further optimizations could be obtained if such a system was scalable, to work in conjunction with conventional techniques (such as DCS) for remote control of the valve actuator operations themselves once remote mode is selected.